Pressure sensitive adhesives may be used with transparent backings, such as for example polyvinyl chloride, polyethylene, polypropylene, polyester and the like, to form transparent labels and decals. Large labels and decals are applied to the sides of trucks, store windows and the like. These surfaces are often squeegied clean prior to the application of the label or decal. This leaves a thin layer of water on the surface. The wet surface allows for the adjustment or positioning of the label or decal. In cases where the backings are transparent, this can result in the development of water-whitening or hazing in the pressure sensitive adhesive, which persists until the system is substantially dry. This drying period may be unacceptably long. For example, a large label, such as that applied to the side of a truck, may take several hours to dry after application of the label. Because the labels and decals are used in advertising and displays, this water-whitening, although only temporary and not detrimental to the pressure sensitive adhesive's ultimate performance, is not aesthetically acceptable.
Although not intending to be bound by theory, I believe that the water-whitening or hazing is caused by the diffusion of water across latex particle boundaries into the interstitial areas in the dried coating forming pockets of water. This gives rise to an osmotic pressure which acts to push the latex particles apart. When the pockets of water become large enough, they scatter visible causing the water-whitening or hazing.
Normally when the aqueous latex emulsion dries, the latex particles coalesce to form a film and the other components of the adhesive formulation which may be ionically-charged and water-soluble become trapped in the interstitial areas of the film. These materials, such as for example, dispersants, surfactants, coalescents, wetting agents, rheology modifiers, thickeners, drying retarders, antifoaming agents, preservatives, heat stabilizers, ultraviolet light absorbers and the like, may attract water because they may be more hydrophilic than the emulsion particles and become the driving force for the migration of the water into the interstitial areas causing the water-whitening problem. Solvent-based adhesives do not experience this water-whitening problem, however they present safety, health and environmental concerns. By contacting the aqueous latex emulsion used in the adhesive formulation or the formulated adhesive with an ion exchange resin, I have found that water-soluble ions may be removed so that the water-whitening resistance or hazing of the pressure sensitive adhesive is improved.